Feature: introduce buffer pool(aka memory manager) to avoid OOM for some opeartors

[xudong963]

Summary

When input data is large, some operators(such as join, sort and so on) will OOM.

It's necessary to introduce a memory manager to avoid OOM

• let users specify memory size to avoid OOM
• Memory manager in charge of spilling for join, sort, group by...
• ...

cc @zhang2014

[FANNG1]

does "some operator" include table scan operator?

[zhang2014]

does "some operator" include table scan operator?

We will use global allocator to track and limit memory usage. the processor level memory trace is still in the todo list.